This invention relates to utilizing the forces by which molten steel ejects impurities from the solidifying crystals during the solidification process in connection with the continuous casting of steel slabs in which molten steel is poured continuously into the open upper end of a chilled mold having a rectangular slab shaped mold passage extending therethrough and in which an embryo casting having a solidified outer shell surrounding a molten core is withdrawn from the lower end of said mold.
As used herein the term "slab" means a casting of indeterminate length having a width which greatly exceeds its thickness. For example, a typical cast steel slab may have a width of as much as eighty inches, more or less, and a thickness of six to nine inches.
Due to the relatively high temperatures of molten steel as compared to non-ferrous metals, and due to the increasingly greater cross section of the slab castings desired by steel companies, it was soon discovered that in continuous casting of steel a much longer cooling period was required to bring the cast strand to a state of complete solidification as compared with the other metals. This necessitated extending the length of the secondary cooling zone, which in turn necessitated a great increase in the overall height of the machines.
Various solutions of this problem were proposed, but a solution which has found general acceptance and is widely used today in the continuous casting of steel is that described in Schneckenburger U.S. Pat. No. 2,947,075.
As therein described, Schneckenburger proposed to use a chilled mold having a curved mold passage extending therethrough so that the embryo casting, as withdrawn from the mold, was curved. The withdrawn curved casting then moved into and through a curved secondary cooling zone and it was then straightened and delivered to a cut-off station, thus reducing the overall height of the machine.
While the Schneckenburger proposal has been successful in solving the height problem, especially for billets, it has created another problem which is particularly troublesome in casting certain steel slabs. Thus, it has been found that during the formation of the shell of the casting within the curved mold passage, the non-metallic inclusions which are found throughout certain types of steel and particularly the aluminum oxides which occur in deoxidized or "killed steel," tend to become trapped in excessive concentration at or near the surface of the casting which is of lesser radius, sometimes referred to as the "inside" surface of the casting, due to the defects in the design of the machine which become evident when the curved mold machine is used to cast slabs, as later explained. On the other hand, it has been observed that such inclusions do not appear so close or so near the surface of the casting which is of greater radius, i.e. the "outside" surface of the casting. Since the condition of the surfaces of steel slabs is highly important in the subsequent working and fabrication of the cast steel, the presence of excessive concentrations of such inclusions at or near the inside surface of the casting is considered to be a serious detriment and it is customary to scarf off or otherwise remove the thin layer of metal on the inside surface of the slab in order to rid it of the objectionable inclusions before further processing of the cast steel is attempted. This difference in the position of the non-metallics in relation to the surface is due to the difference in cooling. That is, the more intense the cooling the greater the distance of the non-metallics from the surface, as is the case at the outside surface of the casting.
It is an object of this invention to avoid the formation of the objectionable concentrations of such inclusions, at or near the inside surface of the casting, by so controlling the cooling and solidification of the solidified shell within the mold that the said inclusions are forced away from the inside surface of the casting and into the interior of the casting where they are less harmful and at the same time avoid the deterioration of the outside surface.